"Say’s Law of Markets answered the fears of under-consumption as the spreading industrial revolution brought forth an ever more bountiful supply of goods. The law’s logic is that production creates the income that is the source of the demand for goods.

In the General Theory, J. M. Keynes recast Say’s Law as the proposition that “the aggregate demand price of output as a whole is equal to its aggregate supply price for all volumes of output.” That formulation would be foreign, if not incomprehensible, to any economist who had ever subscribed to Say’s Law. That law negates the concept of “output as a whole”

The classical economists typically articulated propositions in terms of long-run comparative statics (Ricardo being the prime example). But all (even Ricardo) also analyzed short-run dynamic processes. In modern parlance, excess demand in some markets and excess supply in other markets were entirely consistent with Say’s Law. And that included inter-temporal demand and supply...."

"John Maynard Keynes never had anything like what we would call a graduate education in economics. He was a brilliant amateur with massive gaping blind spots in his economic education — Friedrich Hayek’s well considered judgment was that Keynes had an incompetent economic education in most all economics outside of Marshall. Unfortunately, American mathematical economists turned Keynes’ economic incompetence into rigid math blueprints and graduate school dogma. Not by accident, loud echos of this incompetence are still heard today in the pronouncements of Paul Krugman and many other leading economic figures. Mr. Keynes incompetence as an economist is nowhere seen more glaringly than in his incompetent botching of Say’s Law, an incompetent misunderstanding which has become dogma for many contemporary economists, who themselves have an incompetent education when it comes to almost any economic idea produced prior to, say, 1985 (about the time history of economic thought was removed from graduate education). Incompetence in the area of Say’s Law stands at the center of current economic policies and debate."